(1) Field of the Invention
The present invention relates to a multi-stage honeycomb heater comprising at least two honeycomb heaters, which can be suitably used, for example, as a preheater disposed in the exhaust system of an automobile engine for purifying the exhaust.
(2) Related Art
Converters for the exhaust of an automobile engine have been researched in order to convert harmful substances such as nitrogen oxides (NOx), carbon monoxide (CO), hydrocarbons (HC) and the like contained in automobile exhaust gases. In recent years, with the intensified regulation for the exhaust of an automobile engine, the conversion of the harmful substances particularly at the engine start-up has become an important technical task.
When the temperature of an exhaust gas is low (e.g. right after the engine start-up), the catalyst composition used in the exhaust system is unable to reach the light-off temperature and consequently is unable to purify the exhaust gas sufficiently. Moreover, a large amount of unburnt hydrocarbons are generated during engine warm-up, as compared to steady operation; as a result, the amount of harmful substances emitted during the engine start-up occupies a large portion of the total amount of harmful substances emitted.
As one means for achieving the above task, an electrically conductive heater capable of generating heat when electric current is applied, is provided in the exhaust system of an automobile, and the heater is energized prior to or simultaneously with the engine start-up so as to quickly heat a catalyst composition loaded thereon to the light-off temperature of the catalyst. In the same manner, a main catalyst element provided downstream of the heater but in the vicinity thereof, may be heated also to the light-off temperature of the catalyst.
For example, Japanese Patent Application Kokai (Laid-Open) No. 295184/1991 applied by the present applicant discloses a resistance adjusting type heater including a honeycomb structure having a plurality of passages, at least two electrodes for energizing the honeycomb structure and a resistance adjusting mechanism such as a slit provided between the electrodes.
National Publication of International Application No. 500911/1991 discloses a heater comprising at least two heaters provided in series, each capable of generating heat upon being energized.
In these heaters, however, all the portions of a honeycomb structure of a heater are not uniformly heated to the light-off temperature of the catalyst composition. The upstream portion, i.e., the exhaust gas inlet side of the honeycomb structure is cooled by the cold exhaust gas of relatively low temperature during the engine warm-up so that the upstream portion does not reach the light-off temperature of the catalyst composition as fast as the downstream portions of the honeycomb structure.
Meanwhile, the heat generated in the upstream portion of the honeycomb structure of the heater is transferred by the exhaust gas to the downstream portion of the structure. Consequently, the honeycomb structure has a temperature distribution along the flow path of the exhaust gas. In this case, a low temperature in the upstream portion loaded with a catalyst composition does not convert the exhaust gas much.
According to the aforementioned Japanese Patent Application Kokai (Laid-Open) No. 295184/1991, the heater having a resistance adjusting mechanism such as a slit in the honeycomb structure can control the heat generation, thereby ensuring locally or entirely heating the honeycomb structure thereof. The reference, however, does not disclose any specific method for eliminating the temperature distribution of the honeycomb structure along the flow direction of exhaust gas.
In the heater disclosed in National Publication of International Patent Application No. 500911/1991, the heating of each constituent heater is controlled using at least two external switches. However, since no consideration is made to any of the electrical resistance and heat capacity of each constituent heater, the temperature decrease of the upstream portion is unavoidable. Moreover, use of external switches and control thereof needs a complicated system.